Cardiac arrhythmias account for some 300,000 deaths in the US annually. Although the inherited arrhythmias comprise a small part of the problem, they have proved crucial as models for examination of more complex arrhythmias and have provided important insight into the specific role played by various ion channels in the control of cardiac excitability. This proposal has four specific aims related to the inherited arrhythmias. 1. To determine the mechanism and subgroup specificity of the frequency dependence of the late Na current in LQT3. This aim tests the hypothesis that the APD frequency dependence in LQT3 is mutation specific. 2. To determine whether a new spontaneous mutation in the Na-channel gene identifies the structural basis for closed state inactivation. We have identified a family with a single amino acid deletion in the Ill/IV inter-domain loop that increases closed state inactivation rate almost 20 fold, open state inactivation rate was unchanged. We shall examine the effect of charge and vicinal residues at this locus on closed state inactivation. 3. To determine the impact of expression of the Na-channel and HERG mutants on electrical activity of native cardiac cells in vitro. The functional effects of many LQT mutations are not well understood from their heterologous expression. This aim will test the hypothesis that the complete functional impact of each mutation requires expression in the environment of a cardiac cell. 4. To determine what Na-channel isoforms contribute to the inward current in Purkinje and ventricular cells We have used single cell PCR to clone a Na-channel isoform from Purkinje cells, its amino-acid sequence is identical to the brain III Na channel. Small changes in the expression or kinetics of this channel would cause substantial APD prolongation and LQTS. The experimental approach involves the study of a) wild-type and mutant channels expressed in mammalian cell b) native cardiac cells transfected with recombinant adenoviruses. Isoform expression will be determined by in situ hydridization. The proposed studies will provide new insights into ion channel function, their regulation and possible basis for cardiac arrhythmias.